1. Field of the Invention
This invention generally relates to electromagnetic linear motors and more specifically to such motors adapted for use with electro-acoustical transducers such as loudspeakers.
2. Description of Related Art
Electromagnetic linear motors produce reciprocating motion along an axis in response to alternating current signals applied to a coil structure lying in a magnetic air gap. The amplitude of such alternating current signals causes the coil to reciprocate in the air gap. There are a wide variety of applications for such electromagnetic linear motors.
Loudspeakers represent one application in which electromagnetic linear motors drive loudspeaker cones. In such applications permanent magnets mount on a motor frame with pole pieces to define an annular magnetic air gap. A voice coil assembly on a bobbin or like structure to position a voice coil in the magnetic air gap attaches to the speaker cone. An alternating current signal applied to the voice coil oscillates or reciprocates the voice coil assembly and the attached loudspeaker cone along a loudspeaker axis. The resulting speaker cone vibrations should vary in accordance with the frequency and amplitude of the applied alternating current signal for accurate sound reproduction.
In recent years it has become desirable to increase the power ratings for loudspeakers in order to produce sound that more closely matches an input signal by minimizing distortion and improving frequency response particularly in the bass frequency range. One approach is building loudspeakers that are physically larger and use larger electromagnetic linear motors. As these motors become larger, they become more expensive to manufacture. Moreover, the availability of components for loudspeaker motors that utilize coil sizes greater than approximately four inches is limited because such components, particularly large magnets and pole pieces, are difficult to manufacture for loudspeaker applications.
Some loudspeakers now use dual tandem voice coils in an attempt to increase power capacity. In these loudspeakers a common bobbin carries two voice coils that ride in two annular magnetic air gaps. These voice coils are stated to operate in a push-pull configuration. It is also stated that the two-segment voice coils allow a high excursion with accuracy and controlled motion.
Other constructions for increasing the power capability of loudspeakers also involve two different voice coils. For example U.S. Pat. No. 5,740,265 (1998) to Shirakawa discloses a loudspeaker unit with a magnet system having dual magnetic air gaps and a vibratory system formed with a cylindrical voice coil bobbin carrying first and second voice coils for use in the dual magnetic gaps respectively. U.S. Pat. No. 5,748,760 (1998) to Button discloses a similar structure in which a magnetic structure includes a neodymium magnet and corresponding pole structures to define an elongated air gap that interacts with two voice coils.
Dual voice coils have also been used for other purposes. For example U.S. Pat. No. 4,176,249 (1979) to Inanaga et al. discloses a loudspeaker with a first magnet structure and voice coil for driving a speaker cone. A second magnet drive and independent voice coil eliminate the effect of reaction forces. U.S. Pat. No. 5,828,767 (1998) to Button discloses a loudspeaker with dual voice coils and a single short-circuited braking coil of one or more turns mounted on the voice coil form midway between the two voice coils. Whenever the voice coil assembly displacement approaches a working limit in either direction, the braking coil enters a corresponding one of two magnetic air gaps and limits motion.
U.S. Pat. No. 4,692,999 (1987) to Frandsen discloses a multi-coil, multi-magnet actuator for reciprocating a read/write head mechanism in a magnetic disk storage system as another electromagnetic linear motor application. In this actuator a bobbin carries two coils in two magnetic fields. This structure constitutes a voice coil motor, or solenoid, in which the two coils are oppositely wound to interact with oppositely directed magnetic fields.
In such electromagnetic linear motors it is important that a voice coil or bobbin not contact any of the magnetic pole pieces defining the magnetic air gap. This is especially difficult in loudspeakers constructed to allow large voice coil excursions in the air gap. In these situations it is necessary either to constrain the motion of the voice coil or to increase the air gap to accommodate any motion of the voice coil bobbin off a central axis. However, prior art approaches introduce other issues. For example, the U.S. Pat. No. 5,740,265 employs spiders proximate each end of the voice coil. While such structures may provide proper alignment, they introduce complexities in the design and assembly of component parts and increase manufacturing costs for such electromagnetic linear motors.
Loudspeakers can be subject to electrical and mechanical failures. For example, voice coils are subject to heating during use. Over time it is possible for the insulation between adjacent turns of a voice coil to melt thereby partially or completely short circuiting the voice coil. Such short circuits change the voice coil impedance and operating characteristics or produce a complete voice coil failure.
Likewise the electrical leads from terminals on a loudspeaker frame to the voice coils are subject to fatigue and breakage due to constant reciprocal motion. If the break occurs close to the voice coil, it may be difficult to repair the voice coil. Heat generated during operation can soften adhesive that bonds the coils to each other and the bobbin, so mechanical forces in the individual windings may then pull the windings apart and off the bobbin. Sometimes dirt in magnetic air gaps creates an undesirable rubbing noise as the coil moves in the air gap. Over time suspension components can become worn and sag, also creating a rubbing action. A speaker cone or diaphragm may become damaged due to water absorption, a physical puncture, or long term stress failure. In recent years it has become an object of certain competitions to produce as much sound pressure as possible from loudspeakers installed in an automobile. These operations are abusive to the loudspeakers and often lead to any of the foregoing.
Conventional loudspeakers generally have integral structures or substructures that make loudspeaker repairs from any one or more of the foregoing failures difficult. Anyone of the foregoing or other failures can only be repaired by requiring a disassembly and reassembly process that is difficult, complex and time consuming. Consequently in many cases loudspeakers that fail are merely replaced at significant expense even though a number of components of the failed loudspeaker are still viable.
Often times it would be desirable to retrofit improved parts that were not available when a speaker was purchased or to exchange components, such as coil assemblies, to convert the speaker from one electrical impedance to another. This would afford the speaker hobbyist or professional the opportunity of fine tuning a speaker for a particular application. However, the same restrictions that preclude repair often preclude such retrofittings or customizations. What is needed is a loudspeaker constructed to facilitate the disassembly, repair and reassembly for replacing defective components or for retrofitting or customizing certain components.
Therefore it is an object of this invention to provide an electro-mechanical linear motor that can be readily disassembled and reassembled.
Another object of this invention is to provide a loudspeaker that can be readily disassembled and reassembled for repair, retrofit or customization.
Still another object of this invention is to provide a loudspeaker system with a dual-magnet, dual-voice coil electromagnetic linear motor that can be readily disassembled and assembled for repair, retrofit or customization.
In accordance with this invention a loudspeaker comprises a loudspeaker basket that suspends a loudspeaker cone for displacement along a loudspeaker axis. A motor frame with a magnet structure defines an annular magnetic air gap centered on the loudspeaker axis. An armature supports the voice coil for axial motion in the annular magnetic air gap. A rigid link extends between the armature and the loudspeaker cone. One end of the rigid link attaches to an adjacent one of the armature and loudspeaker cone by a releasable coupling whereby the rigid link can be detached from the adjacent one of the armature or loudspeaker cone.